Thursday, August 28, 2014

Cervelo S5 and the aero road bars

At EuroBike, Cervelo made two huge product announcements, a new S5 aero-road bike, and a revised on the super-premium "would be fun to ride but I can't comprehend paying $10k for a frame and fork" "Project California" RCa.


The S5 is an interesting case. Cervelo has a range of aero road bikes: the S2, S3, and S5. The S2 and S3 are the same frame but with different part specs. These were designed for the more average rider/racer, with an aero front end combined with the rear triangle which is basically the same as the R-series road frames. The S5, on the other hand, was designed with the racer philosophy that, as Specialized tweets, #aeroiseverything. The idea is that racers, looking for all marginal advantages, would ride the S5, and give up a bit of rear-triangle comfort.

Surprise, surprise. On the Garmin pro team, the S3 got much more use. Racing is a complex dynamic, much different than riding solo in the wind. The riders felt better on the S3 with a bit more compliance than the S5.

And so it was inevitable a revision of the S5 was coming, and soon. So this year at EuroBike, they revealed a new version.

Many were expecting an up-rated version of the S3, an aero-road hybrid. But instead, Cervelo led with the aerodynamics, improving the aerodynamics further of the previous S5 while claimin to have improved the stiffness and comfort at the same time yet without resorting to the less aerodynamic rear-triangle features of the S2-3.

Here's the sources of wind resistance on the bike, according to a BikeRadar report. Initially I thought this represented the sources of improvement, but it was pointed out to me on SlowTwitch Triathlon forum it's more likely total resistance. Defining how each component contributes to total resistance is tricky, since parts interact. In a tandem does the front rider contribute more than the rear rider since the rear rider drafts behind the first? Suppose the rear rider is tucked right behind the captain, in virtually perfect draft. Do I conclude the rear rider has no effect on wind resistance? No -- that would be an error. If I were to completely eliminate the front rider, rendering his wind resistance to zero, the total wind resistance of the bike would barely change: the wind which had been hitting the captain would now hit the stoker instead. So the system is nonlinear: the total is not the sum of its parts.

1% – seatpost 
2% – rear brake 
3% – front break 
5% – rear wheel 
9% – drivetrain 
9% – bottle 
9% – fork 
16% – frame 
16% – front wheel 
30% – handlebar

I was initially shocked the handlebar was listed as responsible for more drag than the frame, but the original S5 has a total CdA of around 0.058 m2 according to Bicycling Magazine data, or 580 cm2. A simple calculation of drag on a handlebar (see later), treating it as a cylinder with air at normal incidence, is 100 cm2 just for the top. If I I assume the drops + the brake levers add an extra 75 cm2 then I'm up to 30% (Damon Rinard of Cervelo clarified that this number includes the brake levers). Of course where the rider grabs the bars there will be no contribution to wind resistance from the bars, however: there's a lot of bar-rider interaction.

Starting with the bottle (9%): they didn't introduce any sort of new aero-shaped bottle. They understand road racers need to use whatever bottles they get handed. Instead they followed the model Litespeed used on its carbon road frames and optimized aerodynamics under the assumption there's a bottle on the downtube. This is an interesting question because at the finish of a race, or in most criteriums, there's often no bottles on the bike. But if you're optimizing the bike for long breakaway then you've got to consider bottles. A quantitative assessment would look at "match burning" episodes from real-race power data. But I think optimizing with a bottle is the way to go.

For the frame, then tweaked transitions from the head tube to the down tube and increased head tube taper.

On the front wheel (and rear), they switched to Hed. Of course you can get Hed wheels with the old S5, as well, if you want to buy them.


The biggest contributor is listed as the handlebar, and for that there was a new $400 aero carbon handlebar. Of course you can buy any handlebar you want and put it on the old S5, but this one comes with the new one. It's slick and aerodynamic, with a claimed savings of 4.4 W, which is apparently at 40 kph based on this BikeRadar article (this was confirmed by Cervelo's Damon Rinard), This corresponds to 7.7 W at 30 mph.

4.4 watts at 40 kph/25 mph corresponds, assuming an air density of 1.214 kg/m3, to a CdA reduction of 52.8 cm2. This makes sense: a standard handlebar has a flat top around 36 cm wide, more or less, neglecting the clamped portion and the transition to the drops, with a diameter of near 26 mm beyond the clamp. The Cd of a cylinder is around 0.95 for broadside impact (Mallack and Kumar, 2014), resulting in a net CdA of 89 cm2 for the top part. If the portion of the rider behind the cylinder can draft the cylinder with 30% efficiency, then that reduces the effective CdA of the bar to around 62 cm2 (if the bar wasn't there, the wind that would have hit the cylinder will hit the body instead: doomed either way). So if I can reduce the wind drag of the top portion of the bar by 80%, I get a reduction of 50 cm2. That's really close for a super-back-of-the-envelope estimate.

Super-simplistic kinetic model: consider a stationary air approximation, with no pressure waves. The handlebar displaces a volume vA or air per unit time, speed v multipled by cross-sectional area A. The air develops a kinetic energy density of 1/2 ρ v2, where ρ is the mass-density. That's total rate of kinetic energy transfer 1/2 ρ A v3. This would correspond to a CdA of 1.

But the air slows. Displaced, it falls behind the handlebar. Eventually the body slams into it. By this time, suppose its velocity had dropped 45%. It now has 30% the kinetic energy it did before. The body now increases the kinetic energy of these molecules back to 1/2 ρ A v3. The total kinetic energy transfer rate has been now (1/2) (1.7) ρ A v3. This is a 70% increase versus if the bar wasn't there. So the bar's effective contribution to wind resistance corresponds to 70% of what it would be without a rider behind the bar.

So basically with 20:20 hindsight it's easy to convince yourself this amount of reduction is "obvious".


Then there's the Zipp bar, formerly the Vuma Sprint, now the $350 SL-70. That claims a force reduction of 6.4 watts at 30 mph. This corresponds to a CdA decrease of 44 cm2, in contrast to 30 cm2 for the Cervelo claim for their bar. We know both used a rider on the bike (Cervelo and Zipp both state this), but there's too many sources of variability to directly compare these numbers, for example the position of the rider and how close his body is behind the bar. Both are in the range of my super-simplistic estimate.

So aero road bars appear to be the real deal: essentially free speed for a modest (on order 100 gram) increase in mass. The Cervelo bar claims 270 grams while Zipp claims 240 grams for their bar, Zipp's bar 70 grams more than the claim for their "SL" round bar.

One slight digression:  the Zipp bars sweep forward on the tops.  I like this.  Ritchey Evolutions, for example, sweep back.  Backward sweep pits you'd wrists in a contorted bend.  Forward sweep yields a more neutral position.  So these bars look comfortable too me.

But which one to choose?

The Zipp claims a bit less aero drag reduction, but that doesn't mean much since it wasn't a 1:1 comparison. It claims lighter mass, but mass claims are notoriously unreliable. And it is selling for $50 less than Cervelo has indicated they'll list their bar for. Overall it seems a win for the Zipp bar but that's not a very reliable win.

Note, though, that while 4.4 watts sounds great, that's out of an aerodynamic drag power of around 266 watts assuming CdA of 0.32 to start with, which saves at most 20 seconds per hour.  This is even smaller than the leg shaving advantage Specialized reported from their wind tunnel on YouTube.  So don't expect any dramatic change from these things.


djconnel said...

adjusted calculations since I realized it was more likely Cervelo's wattage saved claims were more likely at 40 kph than 30 mph. This increases time saved by the bars from 11 seconds to 20 seconds per hour, and pushes the Cervelo claim above Zipp's: 7.7 versus 6.4 W at 30 mph.

djconnel said...

Damon Rinard of Cervelo provided several clarifications. One: the watts saved quote was at 40 kph. Quoting at 30 mph or 50 kph provides a more impressive number, and could be defended on the grounds that this speed is often encountered during surges in races, but Cervelo as always is relatively conservative in their claims. The other was that the handlebars included brake levers according to their convention. So that makes the 30% contribution by "handlebars" easier to understand.

fishboy said...

Nice work Dan. @Cervelo (Damon) - please work some aero bar mount points into that aero road bar for triathlon ITU style racing.